When the ENTER INVOICE NO entry screen appears:
a. Key in the Invoice number and press the 
  key, 
 or simply press the
  key, 
 or simply press the 
  key 
 to leave this blank.
  key 
 to leave this blank.
The Invoice number can be up to seven characters long and any combination 
 of alphabetic and numeric characters (punctuation and special characters 
 are not allowed). ( 
 Entering alphabetic characters and punctuation )
Go back to the transaction.